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    Re: DR thread from Nov-Dec '04
    From: Doug Royer
    Date: 2005 Jan 17, 17:12 -0800

    Bill,
    My main source was being taught(made aware of)this in classes when I studied
    to sit for my ABS exam and later my 3rd mate exam.It was shown to me while
    still an ABS by more than 1 officer in actual situations while plying
    oceanic waters.It has served me well(as a rule of thumb)over the course of
    my sea going career.There are many veriables.Some of the resulting
    calculations get complex.As when the officer/helmsman have to hold the ship
    on track and calculate what I mentioned in the origonal post.Wind
    currents,leeway,oceanic currents,bow shear,screw torgue, screw slip and
    other variables must be vectored to obtain the desired corrections to stay
    close to the trackline.When engaged in commerce shipping time is money and
    money is time.So,getting this right is important as the captian or 1st
    officer wouldn't be pleased to find the vessel is x nm off the track line at
    the end of a watch,thus taking longer to get the cargo into port(it could be
    bad enough that the vessel would miss the correct tide and have to ride it
    out outside the harbor and enter the harbor on the next tide).Bad JuJu for
    the OOW if this happens!
    Some one(on this list)once mentioned that I must keep good or exceptional
    DRs when we were discussing some topic.Damn right I did!I didn't like
    getting chewed out by my superior officers for mistakes!
    This is a great rule of thumb and is suprisingly accurate when applied.
    There are a few books that study this and give examples but I've never seen
    it outside of a book not geared for the professional.
    "The Merchant Marine Officer's Handbook" 4th or 5th edition devotes space to
    it in the chapters dealing with shiphandling and pilotting.I haven't seen a
    6th edition yet.
    Other than that one book perhaps in other marine teaching institution's
    sylabus as that is where I was 1st exposed to it.
    
    
    Doug
    
    I've read on the subject in several different texts but had little blue
    water opportunity to actually try it.  Some add variables such as fetch,
    velocity, and how long the wind has been coming from the same direction
    (minimum 12 hrs?) to questimate the velocity of the current and how far off
    the wind direction it is shifted (up to 45d) by the Coriolis Effect.
    Wondered what your source(s) is, and if it/they mentions the above fuzzy
    variables?
    
    Thanks
    Bill
    
    
    

       
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